I have a large 27 uk gallon aquarium and after prolonged and intenseive treatment for whitespot my tank is now forever cloudy.
Someon said I may have killed off the bactiria in my undergravel filter ,if so is ther a way to rapidly re-establish it as my fish are not fond of the awful water.

Some parasitic medications (and certainly antibiotics if you used those) can kill off the good bacteria. You can buy good bacteria to reseed your biological filter. In the mean time, test your ammonia and nitrite readings. If they are high, do water changes. It may even be necessary to do 50% changes daily to keep the levels safe. The white cloud is good bacteria trying to grow quickly without a substrate. Give them a substrate by adding more biological filter media such as cell pore, sponges, polyester floss, bioballs, biowheels, biothings of any kind or really anything with lots of surface area, preferably in the filter itself. Since you have an undergravel filter (UGF), that would mean it should grow on the gravel itself. I much prefer hang-on-tank (HOT) filters over UGF's. I suggest buying a HOT filter with a biowheel. You can still run the UGF too.

ok ,since i last posted the tank has started to clear ,,a friend recommended that I tried a product called filter aid that forces the cloud to clump together and get sucked in to the gravel easier.
I might try a box on the side of the tank though I find them unsightly

I think of the hang-on-tank filters as little waterfalls but to each his own. AccuClear will also cause the cloudiness to clump. With either chemical, it's a temporary fix (the cloudiness will return usually). Good luck!

My tank is still cloudy the only way I can descride the tank now is 'white cloud', the water seems clean otherwise ,I try not to put too muck food in now as well.
I have been told Imight have Bacterial Boom in my tank .
The solution I am thinking of is radical ,totaly drain the tank and clean it right out and start a fresh.
As this a last ditch attempt to sort the problem out I was wondering if there is any other way to forch the bacteria to stay in the gravel ,and thus give me a clear tank.

What animals do you have in the tank? Is it overcrowded? It does sound like a bacterial bloom. They will go away if you have enough surface area on which the good bacteria can grow. I still suggest adding a hang-on-tank filter with a biowheel or biological substrate with it. If the tank is overcrowded, it will never clear up. Tearing down the tank will help at first but in the long run, it may make things worse if good bacteria are killed in the cleaning. Good bacteria will grow on the gravel but will grow better if there's a lot of oxygen down there, and the gravel is not allowed to stagnate. To prevent that, use a gravel vacuum on the gravel every week to stir it up and the gunk and gases removed with a water change.

My section on cloudy water (it's not much):
http://www.fishpondinfo.com/plants/algaeinfo.htm#cloudy

I have 2 large fantail gold fish ,they are going to a new home soon,6 mollies,4 guppies ,3 redeye tetra,2 danios,2 large plecos ( not fed alot) and almost 20 platies .The tank is about a meter long and over a foot tall and over a foot deep (27/28 UK gallons) .
I can't really afford a hang on filter at the moment ,the water is not getting worse ,but it's not getting clearer either.

thats about 33-34 US gallons, i'd say that tank is extremely overcrowded...2 large fantails, i am imagining about 4 inches each, plus the tropical fish are about 43 inches worth, plus 20 platies??? babies or adults, if adults thats like another 50 inches of adult fish, your looking at 100+ inches of fish in a 34 gallon tank.

Yep, sounds overcrowded. The water won't improve much under those conditions. If you think 4" is a large fantail, you should see that photo of one that was almost 2 feet long that circulated a few years ago!

How many fish should I have ?
The gold fish are about 4" nose to tail but they are going to a new home,I am going to humanly kill 1 molly that will not get well and is not happy two danois that have dropsy and a guppy with the same disease, But I take it in need to have alot less so how many more must go ?

I would never say to get rid of (kill) any fish. If you can't find new homes for some, get more aquariums or ponds, etc., then they can be kept overcrowded although not ideal. Frequent partial water changes would help keep the system going.

The tank is about 32 US gallons. Such a tank should have no more than 32 inches of fish (ideally less). Without the goldfish and plecos, you probably have about 40 inches of fish which is manageable. How big are the "large" plecos?

The bigger one is about 10" and the smaller one is 8".
I would just like to add I am not killing my fish to get rid of them ,they are old and suffering with dropsy,and even when I am doing them a favour I am reluctant to kill them.
I do have two 12 uk gallon fry tanks that are all ready crowded one with 20 + guppys and 15 molly babys and two small plecos ( all are small no bigger than 1.5 cm ) and one with 40 + assorted mollys and a small pleco.
These tanks are fine both have hang on filters and regular gravel cleaning.
I can't relly transfer any from the bigger tank as it is still suffering with whitespot a bit though it is on the way out.

Hi I would like to say that my tank is finally on the mend the water is finally starting to clear. I have been cleaning my gravel in in sections thus allowing the bacteria to colonise in clumps.
As I remove water now it does not get worse .
Hopefully this wil not happen again my fish are finally feeling happier .

I'm glad things are improving. Sometimes it takes a while for cloudy water to go away.